Knife sharpener



FIG. 1 is a perspective view of a knife sharpener showing my new design;

FIG. 2 is a top plan view thereof;

FIG. 3 is a front elevational view thereof;

FIG. 4 is a rear elevational view thereof;

FIG. 5 is a left side elevational view thereof; and,

FIG. 6 is a right side elevational view thereof. 

The ornamental design for a knife sharpener, as shown. 